Process for gasifying caking coals



Aug. 26, 1969 FORNEY ET AL 3,463,623

' PROCESS FOR GASIFYING CAKING GOALS Filed Sept. 7, 1967 Cool load 7 /4 Sham Productqos and czyqnn Separator Pretreatment zone Ash Gosifyinq zone INVENTORS Albert J. Forn ey Stanley J. Gosior Joseph H. Field Ernest S. Cohen ATTORNEY United States Patent 3,463,623 PROCESS FOR GASIFYING CAKING COALS Albert J. Forney, Coraopolis, and Stanley J. Gasior and Joseph H. Field, Pittsburgh, Pa., assignors to the United States of America as represented by the Secretary of the Interior Filed Sept. 7, 1967, Ser. No. 666,565 Int. Cl. Cj 3/06 US. Cl. 48202 7 Claims ABSTRACT OF THE DISCLOSURE A methane-rich synthesis gas is produced from finely divided particulate caking coals by subjecting the coal to a pretreatment in the presence of oxygen and steam in a free-fall zone surmounting and in open communication with a fluidized bed in which gasification occurs.

This invention resulted from work done by the Bureau of Mines of the United States Department of the Interior, and the domestic title to the invention is in the government.

BACKGROUND OF THE INVENTION It is well known to convert coal to synthesis gas using the Lurgi process. In this process, a particulate coal bed is reacted with an oxygen and steam mixture to produce a product gas composed primarily of hydrogen, carbon dioxide, carbon monoxide and methane. The conventional Lurgi process however is limited to a non-caking coal feed having a particle size of about A; inch minimum. If a caking coal is used as a Lurgi feed, its caking properties must be destroyed in a pretreatment or carbonization step in order to prevent agglomeration or fusion in the Lurgi generator.

It is also known to gasify finely divided particulate coal in a fluidized bed. Typical of such processes is that disclosed by the Kalbach patent, US. 2,662,816. Like the conventional Lurgi process, the fluidized bed gasification processes cannot utilize caking coal feeds without a feed pretreatment step to destroy the caking properties of the coal.

In order to avoid particle agglomeration when using a caking coal feed, Kalbach resorts to a fluidized bed pretreatment step to degasify and carbonize the coal feed. Such a pretreatment avoids agglomeration in the gasifier but introduces process complications. Two fluid beds in series are difficult to operate. Any surge in the pretreater throws raw untreated coal into the gasifier creating problems of agglomeration and loss of fluidization.

Higher reaction rates may be attained in the fluidized bed gasification of coal as compared to the conventional Lurgi process. These faster rates are probably due primarily to the increased coal surface area available for reaction. Fluidized bed gasification processes also are advantageous as compared to the Lurgi process in that a fluidized bed gasification reactor is vastly simpler than is a Lurgi converter. However, a fluidized bed process results in a product gas having very low concentrations of methane.

There is no one-step process for the gasification of carbonaceous materials which will produce a gas of sufiiciently high Btu content to qualify as a pipeline gas; that is, a gas having a heating value of 800 to 1000 B.t.u. per s.c.f. One-step processes, such as the Lurgi, produce a gas having a maximum heating value of about 450 B.t.u. per s.c.f. Consequently, it is necessary to perform a second step of after treatment and upgrading to produce a gas of acceptable heating value.

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The conventional upgrading step includes the removal of inerts such as carbon dioxide in combination with a methanation reaction in which hydrogen and carbon monoxide form methane. Methanation includes an adjustment of the molar ratio of hydrogen and carbon monoxide to about 3 to 1 which is accomplished by the water gas shift reaction. The methane-tion reaction can be represented by the formula:

Thus, four volumes of feed gas react to produce one volume of methane. By making a synthesis product gas which is originally rich in methane, the volume of gas handled in the water gas shift and methanation reactions is drastically reduced with the attendant advantages of smaller equipment and lower process cost.

The process of this invention produces a methane-rich synthesis gas from a swelling and agglomerating carbonaceous feed without a separate pretreatment step. In addition to simplifying the apparatus required in carrying out the process, the product gas produced has a high methane content.

It is an object of this invention to produce a methanerich synthesis gas from a carbonaceous material having swelling and agglomerating properties.

Another object of this invention is to pretreat and gasify a finely divided caking coal within the same reactor.

It is a further object of this invention to produce a high methane content synthesis gas using a fluidized bed gasification process.

DESCRIPTION OF THE INVENTION Referring now to the figure, reference number 1 indicates generally a vertical cylindrical retort having an upper free-fall pretreating zone 2 and a lower fluidized bed gasification zone 3. A relatively cool or cold pulverized carbonaceous material such as a highly caking bituminous coal is supplied by hopper 4 through feeding conduit or duct 5 containing a suitable feeding and controlling device 6. The feed hopper 4 is preferably a lock hopper pressurized to the retort operating pressure with air, inert gas or product gas. Feeding device 6 comprises a rotary valve powered by a variable speed drive but any controllable rate solids feeder such as a screw conveyor may also be used.

An entraining and treating gas comprising oxygen and steam is introduced into the feeding duct 5 via conduit 7. Duct 5 extends downwardly into the free-fall pretreatment zone 2 for a sufficient distance, preferably about one-third of the length of the pretreatment zone, so that the entering coal is entrained and partially preheated, The treating gas-entrained coal feed as it leaves the end of the feeding duct contacts an ascending hot gas stream from the fluidized 'bed gasifier 3. Cooling means 8 surrounding the pretreatment zone provides control of gas temperature in the zone and also serves to recover process heat. These cooling means preferably comprise a water jacket having an entry conduit 9 and exit conduit 10. While water and/or steam is preferred as the cooling medium, other heat transfer fluids may be used.

Suflicient cooling capacity must be provided so as to cool the hot reaction gases from the gasifier to a temperature just below the initial softening point of the coal feed at the point where the coal feed enters the pretreatment zone. If the ascending gas temperature exceeds the softening point of the coal at the feeding duct exit, then fusion and agglomeration of the coal particles results. The initial softening temperature of highly caking bituminous coals varies generally in the range of about 360 C. to 475 C. For example, a highly caking bituminous coal such as that from the Pittsburgh seam 3 has a softening temperature of approximately 400 C. When using this particular coal in the process, the pretreatment zone is preferably controlled so as to provide a gas temperature at the feeding duct exit of about 375 to 400 C.

Gas velocities in the pretreatment zone must be below the entrainment velocity of the coal feed so as to allow the coal particles to progress downwardly countercurrent tothe ascending hot gases. Exposure to the hot gasification products and the oxygen and steam of the treating gas tends to devolatilize and carbonize the coal feed and thus destroy its caking properties.

Gasification zone 3 is directly below and in open communication with pretreatment zone 2. Gas velocity in zone 3 is maintained sufiiciently high to fiuidize the coal particles entering from the pretreatment zone. Relative gas velocity in the two zones is controlled by the area ratio of the two zones. Generally, the area of the pretreatment zone must be at least twice that of the gasification zone. Using a coal feed sized 70% through 200 mesh, it has been found that an area ratio of the pretreatment zone to the gasification zone of about 4 to 1 provides satisfactory results.

A reactive gas mixture of oxygen and steam is supplied to the bottom of gasification zone 3 via conduit 11. The gas mixture fluidizes and reacts with particulate coal in the zone to produce products comprising hydrogen, carbon monoxide, carbon dioxide and methane. These products, along with excess steam, ascend through the pretreatment zone 2 and are removed from the reactor via conduit 12. The product gas is then passed through a separator 13 to remove entrained solid particles, comprising mostly ash, from the gas stream and discharge them from the system via line 15. Cleaned gas is discharged through line 14. The separator 13 may be for example a centrifugal type or may comprise ahigh temperature filter. Because of their higher density, ash particles tend to concentrate in the lower portion of the fluidized bed maintained in gasification zone 3. Excess ash is removed from the bed by screw-type ash extractor 16.

The mixture af gases discharged through line 14 may be treated to remove tars, sulfur compounds and carbon dioxide. After purification, the gases may be subjected to the conventional water gas shift reaction followed by methanation to produce a methane-rich pipeline gas.

While the apparatus and process illustrated by the figure represents a preferred embodiment, other treating gas and coal feeding arrangements have been found satisfactory. For example, the pulverized coal may be fed into the top of the pretreatment zone while introducing the steamoxygen treating gas mixture at the base of that same zone. This arrangement produces results equivalent to those of the preferred embodiment but requires the use of 20 to 40% more oxygen in the pretreatment step.

Gasification with oxygen and steam in the fluidized bed gasification zone is carried out at a temperature within the general range of 800 to 1000 C. and at a pressure of about 2 to 40 atmospheres. Steam is introduced in the bottom of the gasifier in the range of 25 to 50 s.c.f. per pound of coal feed and oxygen requirements for the gasifier are in the range of 3 to 6 s.c.f. per pound of coal feed. While air may be used to supply the oxygen requirements for the gasifier, it is preferred to use a relatively pure oxygen in order to avoid nitrogen dilution of the product gas. Preferred operating conditions are those which maximize the production of methane in the gasifier and include a temperature of about 900 C. and a pres sure of about 20 to 25 atmospheres. Oxygen requirements in the pretreatment zone vary from about 0.5 to about 1.5 s.c.f. per pound of coal feed. It is preferred to operate in the range of about 0.7 to 1.0 s.c.f. oxygen per pound of coal. Steam introduced into the pretreatment zone in admixture with the oxygen serves as an oxygen diluent, as an entraining and transporting gas for the coal and serves to partially preheat the coal. It has been found advantageous to use from 10 to 20 s.c.f. steam per s.c.f. oxygen in the pretreatment zone.

In the carbon-water-oxygen equilibrium system, methane production increases with pressure but decreases as reaction temperature rises. The gasification zone is preferably operated at conditions which maximizes methane production while maintaining a high reaction rate. In this process, hot gases after leaving the gasification zone continuously contact descending coal particles at progressively lower temperatures; a situation which favors the production of additional methane. 1n the free-fall or pretreatment zone of the reactor, the raw coal feed is substantially devolatilized which produces additional quantities of methane. As a result, the product gas leaving the reactor contains about 15 to 20% methane which is far greater than that customarily produced in a fluidized bed gasification.

The following is an example of the application of the process to the gasification of a highly caking coal.

EXAMPLE Coal from the Pittsburgh seam of the Bruceton mine having a free swelling index of about 8 was crushed to a particle size such that 70% passed through a 200 mesh screen and all particles coarser than 35 mesh were removed. The coal was fed into a reactor such as that illustrated in the drawing in which the height of the free-fall pretreatment zone was 10 feet. The feed duct extended 3 feet downwardly into the pretreatment zone. Temperature of the gas at the exit of the feed duct was maintained between 375 and 400 C. Steam at 375 C. in an amount equivalent to about 15 s.c.f. per pound of coal along with about 0.7 s.c.f. oxygen per pound of coal was introduced into the feed duct to entrain and treat the coal feed. Area of the pretreatment zone was four times that of the fluidized bed gasification zone.

The gasification zone was maintained at a pressure of 20 atmospheres and a temperature of 900 C. Oxygen in the amount of 4 s.c.f. per pound of coal feed along with 35 s.c.f. steam per pound of coal was introduced into the bottom of the gasification zone to fiuidize and react with the coal. An average of a number of runs showed product gas yield on a nitrogen and carbon dioxide free basis to be approximately 23 s.c.f. per pound of coal feed and analyzing from 15 to 20% methane.

While the specification and example illustrate the use of a caking bituminous coal feed, other carbonaceous materials such as low temperature petroleum coke which display similar softening and agglomerating properties may be used in the process.

It will be understood that a number of variations and adaptations of the disclosed invention are possible without departing from its spirit or scope.

What is claimed is:

1'. A process for the gasification of a finely divided carbonaceous material having softening and agglomerating properties when heated which comprises:

(a) feeding said carbonaceous material downwardly into the upper portion of a vertical reactor, said reactor having an upper zone of relatively low gas velocity and a lower zone of relatively high gas velocity, said upper zone surmounting and being in open communication with said lower zone;

(b) contacting said descending carbonaceous material in said upper zone with a hot ascending gas stream, the velocity of said gas stream being insufficient to entrain said carbonaceous material and the temperature of said gas stream at the point of introduction of said carbonaceous material being slightly less than the initial softening temperature of said carbonaceous material;

(c) introducing a treating gas into the reactor in the proximity of carbonaceous material introduction, said treating gas comprising at least about 0.5 s.c.f. oxygen per pound of carbonaceous material feed;

(d) introducing a reactive gas into the lower zone of said reactor, said reactive gas comprising about 3 to 6 s.c.f. oxygen and 25 to 50 s.c.f. steam per pound of carbonaceous material feed;

(e) controlling the velocity of said reactive gas in said lower zone so as to maintain a fluidized reaction bed of said particulate carbonaceous feed material within said lower zone;

(f) maintaining the temperature of said fluidized reaction bed within the range of about 800 to 1000 Q;

(g) maintaining the pressure within said lower zone within the range of about 2 to 40 atmospheres; and

(h) removing from said upper reaction zone a methane-rich product gas stream.

2. The process of claim 1 wherein said carbonaceous material comprises a highly caking bituminous coal.

3. The process of claim 1 wherein said treating gas comprises about 0.5 to 1.2 s.c.f. oxygen per pound of coal feed in admixture with about 10 to 20 s.c.f. steam per s.c.f. oxygen.

4. The process of claim 1 wherein the temperature of said fluidized reaction bed is maintained at about 900 C. and the pressure within said lower zone is maintained within the range of about 20 to 25 atmospheres.

5. The process of claim 1 wherein said carbonaceous 7.. The process of claim 6 wherein the temperature of said hot ascending gas stream is in the range of 375 to 400 C. at the point whereat said admixed carbonaceous 10 feed and treating gas is introduced into said upper zone.

References Cited UNITED STATES PATENTS 2,637,683 5/1953 Kassel 201-38 XR 2,662,816 12/1953 Kalbach 48202 2;743,2l7 4/ 1956 Silsby 48--2O2 XR 2,776,879 1/1957 Gumz 48202 2,782,109 2/1957 Roberts 48202 XR MORRIS O. WOLK, Primary Examiner R. E. SERWTNG, Assistant Examiner US. Cl. X.R. 

